The invention relates to the manufacturing of containers by blow molding or stretch blow molding from preforms made of thermoplastic material such as PET (polyethylene terephthalate).
The manufacturing of the containers is conducted within a dedicated installation and comprises two main phases: heating and shaping. The preforms produced from molding (generally by injection, although there are techniques of molding by compression) are initially stored in bulk in a hopper of a supply system, from where they are extracted and then oriented (generally neck down) to be heated.
For the heating phase, the installation comprises a furnace equipped with infrared radiation sources (typically halogen lamps) by which the preforms from the hopper pass in a line. The heating phase is cyclic, with the preforms intended for a given container range being exposed to the same radiation for the same predetermined duration (commonly called heating time).
A complete heating cycle, which extends from the intake into the furnace of a cold preform, exiting the supply system, to the discharge from the furnace of the same hot preform, generally lasts less than one minute, typically about 20 seconds.
In theory, all of the preforms that have followed the same heating cycle have, at the outlet of the furnace, the same temperature, which depends essentially on the speed of advance of the preforms, the length of the furnace, the spectrum and the power radiated by the latter, as well as absorption properties of the material of the preforms.
In an ordinary furnace (halogen), the spectrum radiated by the sources is fixed. The same is true of the length of the furnace. The absorption properties of the material also cannot be controlled. By contrast, the speed of advance and the radiated power can vary based on instructions programmed into an electronic (or computer) control unit of the furnace.
If the theoretical temperature that the preforms are to have at the outlet of the furnace is referred to as “temperature instructions,” these instructions in reality arise from the power radiated in the furnace, and therefore from the electrical power delivered to the sources, which can be programmed into the control unit. As a variant, it is possible to program temperature instructions directly, the control unit then being programmed to convert this temperature into power to be delivered to the sources. Below, the expression “temperature instructions” just as well refers to a theoretical temperature value programmed into the control unit or a power value to be delivered to the radiation sources.
For the shaping phase, the installation comprises a rotary-type shaping unit, provided with a carrousel and equipped, on the periphery of the carrousel, with a number of molds bearing the imprints of containers to be shaped.
At the outlet of the furnace, each preform that is still warm is inserted into a mold. A fluid (in general, air) is injected under pressure into the preform to shape the container. The shaping generally comprises two steps: a first so-called pre-blow-molding step during which the fluid is injected at a (relatively low) pressure and a pre-blow-molding flow rate that are predetermined, immediately followed by a blow-molding phase, during which the fluid is injected at a (comparatively higher) pressure and a blow-molding flow rate that are predetermined. Typically, for the production of containers intended to be filled with plain water, the pre-blow-molding pressure is on the order of 7 bars, and the blow-molding pressure is on the order of 25 bars.
The pressurized pre-blow-molding fluid is produced from a pressure source; it is delivered via a solenoid valve controlled by an electronic (or computer) control unit of the shaping unit. The pre-blow-molding pressure can be adjusted by means of a pressure regulator. The same is true of the flow rate, which can be adjusted by means of a flow rate restrictor. It is consequently understood that the pre-blow-molding pressure and the pre-blow-molding flow rate can be adjusted according to the respective instructions programmed into the control unit of the shaping unit, which directs the solenoid valve, the pressure regulator, and the flow rate restrictor based on the instructions.
The advances realized during the last decade in the fields of sensors and computer science have allowed manufacturers to outfit the installations to ensure a monitoring of certain critical parameters (in particular the heating temperature of the preforms and the pressure in the containers during shaping) and to carry out an analysis of these parameters to detect therein the characteristic derivatives of machine defects.
The measurement of temperature is illustrated in the French patent application FR 2 935 924 and in its U.S. equivalent US 2011/0236518. The measurement of pressure is illustrated in the French patent application FR 2 909 305 and in its U.S equivalent US 2010/201013.
In this latter application, it is recommended that a local pressure peak be detected during the pre-blow-molding phase, and, when this peak (reached at the time of development of the preform where the plastic flow threshold of the material is reached) does not coincide with a theoretical peak, that at least one machine parameter from among a set comprising in particular the heating temperature, the pre-blow-molding pressure, and the pre-blow-molding flow rate be modified. In other words, the installation is programmed to detect its own defects and to correct them if necessary, so as to limit the manufacturing defects in the containers.
This process operates wonderfully as long as the noted derivative of the local pressure peak is effectively due to a machine defect. In this case, the installation can, by a correction of its own parameters, bring the local pressure peak back into a tolerance zone in which it is accepted that the containers that are produced are true to the model.
In certain cases, however, it was noted that these corrective actions did not result in either eliminating the derivative of the local pressure peak or even controlling it. On the contrary, it happened that these corrective actions caused a worsening of the derivative of the peak, in contrast to the anticipated effect, to the point that it became necessary to stop production to reset the parameters of the machine.
Research has been conducted for identifying the causes of the derivative affecting the local pressure peak independently of the machine parameters. This research made it possible to determine that the local pressure peak can be assigned in a significant manner by a variation in the moisture level in the preforms.
To the knowledge of the inventors, such a relationship had never been demonstrated. It is certainly known that the mechanical properties of a plastic material (quite particularly the PET) can be affected by the moisture level of the material. However, as L. Vouyovitch van Schoors explains it in Vieillissement hydrolytique des geotextiles polyester (polyéthylène téréphthalate)—{acute over (E)}tat de l'art [Hydrolytic Aging of Polyester Geotextiles (Polyethylene Terephthalate)—State of the Art], in Bulletin des Laboratoires des Ponts et Chaussées [Bulletin of the Laboratories of Bridges and Highways], No. 270-271, October/November/December 2007, “it is the chemical aging and more specifically the phenomena of hydrolysis that govern the durability of these materials ( . . . ). This type of aging is generally very slow at ambient temperature because of the low speed of the elementary chemical treatment and a slow diffusion of water within the polymer matrix.”
In the case of shaping containers from preforms made of PET, it is not very likely that the variation in the local pressure peak during the pre-blow-molding phase can result in a phenomenon of hydrolysis of the material, because the average storage periods of the preforms (at ambient temperature) before their introduction into the manufacturing chain of the containers are much too short: several days, a month at the very most.
By contrast, it is plausible that a difference in moisture level of the material from one preform to the next becomes reality during the heating by a difference in their respective mechanical properties, inducing a different behavior of these preforms starting from the pre-blow-molding.
More specifically, it is evident from the tests conducted on preforms having various moisture levels that the strength of a container varies inversely to the moisture level of the perform from which it is produced. It is therefore important to ensure that the moisture level of the preforms does not exceed a determined threshold, beyond which the containers have an inadequate strength and should be scrapped.
In practice, however, the manufacturing rates (on the order of 50,000 containers per production line) do not make it possible to perform a systematic measuring of the moisture level of the preforms. At the very most, it is possible to take samplings and perform manual measurements on the sampled preforms. An ordinary technique for measuring the moisture level of a plastic consists in reducing a sample made of powder or granules (the measurement being consequently destructive), in carrying out a first weighing thereof, then in heating it under a dry atmosphere for a predetermined duration (on the order of several minutes), and in carrying out a second weighing thereof, with the difference in weight making it possible to determine the evaporated water weight. A program makes it possible to deduce therefrom the total water weight initially present in the sample, in relation to the total weight of the latter.
This technique, illustrated by the European patent application EP 2 574 902 or its U.S. equivalent US 2013/081454, requires an at least partial destruction of the sample and an extension of treatment of several minutes. For each of these two reasons, such a technique cannot be applied to the continuous check of the moisture level of the preforms in a container production line. First of all, the integrity of each preform is to be preserved, unless it is imagined to design a new type of preform provided with a detachable part intended for the check, which assumes a substantial modification of the manufacturing technique as well as an increased consumption of material. Next, a manufacturing rate of 50,000 containers per hour means that a preform is introduced into the production line (respectively, a container is evacuated from the production line) approximately every 70 milliseconds. Only an optical measurement would keep up such a pace. Tests, however, have demonstrated that a variation in the moisture level does not have a measurable effect either at the inlet of the furnace (no measurable variation in the transparency of the material at the same temperature) nor at the outlet (no measurable variation in the heat distribution in the preform under the same heating conditions).